Surface plugin update availability on the plugins page

The plugin manager page already showed each installed plugin's version and
had an Update button, but nothing told users an update actually existed —
they had to guess. The store manager already compares the installed
manifest version against the registry's latest_version for its reinstall
decision; this surfaces that same signal in the UI.

- api_v3 /plugins/installed now returns `latest_version` (from the registry
  cache, no extra network call) and an `update_available` flag computed by a
  new semver-aware helper `_is_plugin_update_available`. A locally modified
  plugin whose version is ahead of the registry is not flagged.
- The installed-plugin card shows "vX.Y.Z available" next to the installed
  version, and the Update button becomes emphasized ("Update to vX.Y.Z" with
  a gentle pulse) when a newer version is published — mirroring the app's own
  update banner styling.
- Added tests for the helper and the endpoint fields.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012EW8yDbsk8EceDpq6Hjgqj

test/test_web_api.py

@@ -372,11 +372,68 @@ class TestPluginsAPI:
         }
         
         response = client.get('/api/v3/plugins/installed')
-        
+
         assert response.status_code == 200
         data = json.loads(response.data)
         assert isinstance(data, (list, dict))
-    
+
+    def test_installed_plugins_report_update_available(self, client, mock_plugin_manager):
+        """Installed-plugin entries surface latest_version + update_available
+        by comparing the on-disk manifest version to the registry."""
+        from web_interface.blueprints.api_v3 import api_v3
+        api_v3.plugin_manager = mock_plugin_manager
+        # No on-disk manifest to merge — keep the version we hand in below.
+        mock_plugin_manager.plugins_dir = '/nonexistent-plugins-dir'
+        mock_plugin_manager.get_all_plugin_info.return_value = [
+            {'id': 'weather', 'name': 'Weather', 'version': '1.0.0'}
+        ]
+        # Avoid touching plugin instances (Vegas hooks, enabled fallback).
+        mock_plugin_manager.get_plugin.return_value = None
+        # Registry advertises a newer version than the installed one.
+        api_v3.plugin_store_manager.get_registry_info.return_value = {
+            'verified': True, 'latest_version': '1.2.0'
+        }
+
+        response = client.get('/api/v3/plugins/installed')
+
+        assert response.status_code == 200
+        payload = json.loads(response.data)
+        entry = payload['data']['plugins'][0]
+        assert entry['version'] == '1.0.0'
+        assert entry['latest_version'] == '1.2.0'
+        assert entry['update_available'] is True
+
+    def test_installed_plugins_no_update_when_current(self, client, mock_plugin_manager):
+        """No update is flagged when installed version matches the registry."""
+        from web_interface.blueprints.api_v3 import api_v3
+        api_v3.plugin_manager = mock_plugin_manager
+        mock_plugin_manager.plugins_dir = '/nonexistent-plugins-dir'
+        mock_plugin_manager.get_all_plugin_info.return_value = [
+            {'id': 'weather', 'name': 'Weather', 'version': '1.2.0'}
+        ]
+        mock_plugin_manager.get_plugin.return_value = None
+        api_v3.plugin_store_manager.get_registry_info.return_value = {
+            'verified': True, 'latest_version': '1.2.0'
+        }
+
+        response = client.get('/api/v3/plugins/installed')
+
+        assert response.status_code == 200
+        entry = json.loads(response.data)['data']['plugins'][0]
+        assert entry['latest_version'] == '1.2.0'
+        assert entry['update_available'] is False
+
+    def test_is_plugin_update_available_helper(self):
+        """Unit-level checks for the semver-aware update comparison."""
+        from web_interface.blueprints.api_v3 import _is_plugin_update_available
+        assert _is_plugin_update_available('1.0.0', '1.0.1') is True
+        assert _is_plugin_update_available('1.0.1', '1.0.1') is False
+        # Local build ahead of the registry must not be flagged.
+        assert _is_plugin_update_available('2.0.0', '1.9.9') is False
+        # Missing either side yields no signal.
+        assert _is_plugin_update_available('', '1.0.0') is False
+        assert _is_plugin_update_available('1.0.0', '') is False
+
     def test_get_plugin_health(self, client, mock_plugin_manager):
         """Test getting plugin health information."""
         from web_interface.blueprints.api_v3 import api_v3